In 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), Orthogonal Frequency Division Multiplexing (OFDM) is adopted as a downlink (DL) transmission scheme.
GPP LTE (Long Term Evolution) standard is the last stage in the realization of true 4th generation (4G) mobile telephone networks. Most major mobile carriers in the United States and several worldwide carriers have announced plans to convert their networks to LTE beginning in 2009. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS). Much of 3GPP Release 8 focuses on adopting 4G mobile communications technology, including an all-IP flat networking architecture.
The 3GPP LTE standard uses orthogonal frequency division multiplexing (OFDM) for the downlink (i.e., from the base station to the mobile station). Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique that transmits on many orthogonal frequencies (or subcarriers). The orthogonal subcarriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects.
In Release 8 LTE systems, a user equipment (UE) or mobile station (MS) is required to perform channel estimation based on common reference signals (CRSs) over the entire bandwidth. Once channel estimation is performed, the mobile station (or UE) performs demodulation based on different transmission modes indicated by the different formats of the downlink control information. For example, when downlink spatial multiplexing is performed, downlink control information (DCI) format 2 is used and the mobile station performs demodulation based on the resource assignment and TPMI (transmission PMI) contained in the DCI format.
In 3GPP Technical Specification No. 36.212, version 8.8.0, “E-UTRA, Multiplexing and Channel Coding” (December 2009), the definition of TPMI is defined in Table 5.3.3.1.5-4 (2 antenna ports) and in Table 5.3.3.1.5-5 (4 antenna ports) of Section 5.3.3.1.5. The 3GPP Technical Specification No. 36.212, version 8.8.0, is hereby incorporated by reference into the present disclosure as if fully set forth herein.
The base station (or eNodeB) indicates to the mobile station (MS) or user equipment (UE) whether the base station (BS) is implementing wideband precoding or subband precoding based on mobile station feedback and the mobile station performs downlink demodulation accordingly.
In LTE-Advanced (LTE-A) systems, the downlink demodulation is based on dedicated reference signals (DRS), which are UE-specific reference signals (UE-RS).
In LTE-Advanced systems, demodulation of the data channel is based on the precoded UE-specific reference signal. That is, the reference signals are precoded using the same precoder as the data channel as described in 3GPP Document No. R1-090529, “Way Forward On CoMP And MIMO DL RS,” Outcome of Ad Hoc Discussions (January 2009), and 3GPP Document No. R1-091066, “Way Forward On Downlink Reference Signals For LTE-A,” (March 2009), both of which are hereby incorporated by reference into the present disclosure as if fully set forth herein.
Reference signals (RSs) targeting PDSCH demodulation (for LTE-A operation) are also UE-specific and are transmitted only in scheduled resource blocks (RBs) and the corresponding layers. Different layers can target the same or different UEs. The design principle is an extension of the concept of Rel-8 UE-specific RS (used for beamforming) to multiple layers. Reference signals on different layers are mutually orthogonal. Reference signals and data are subject to the same precoding operation, and complementary use of Rel-8 CRS by the UE is not precluded.
In Document No. R1-094413, “Way Forward On The Details Of DCI Format 2B For Enhanced DL Transmission,” 3GPP RAN1#58bis, Miyazaki (October 2009), which is hereby incorporated by reference into the present disclosure as if fully set forth herein, an agreement has been made for DCI format 2B. In the agreement, the DCI Format 2B is based on DCI Format 2A. One (1) bit is added for the source channel identifier (SC-ID) and the Swap Flag is removed. For rank 1 transmission, the new data indicator (NDI) bit of the disabled transport block is re-used to indicate port information. A value of 0 is used to indicate an enabled transport block (TB) associated with port 7. A value of 1 is used to indicate an enabled transport block associated with port 8. For rank 2 transmission, TB1 is associated with port 7, and TB2 associated with port 8. DCI format 2C can be constructed based on DCI format 2B for Release 10 transmission modes for facilitating dynamic SU- and MU-MIMO switching.
Since an eNodeB could potentially perform resource block (RB)-based precoding, the baseline granularity for channel estimation and demodulation is one resource block (RB). However, as disclosed in 3GPP Document No. R1-093105, “UE-RS Patterns for LTE-A”, Qualcomm Europe (August 2009), which is hereby incorporated by reference into the present disclosure as if fully set forth herein, “resource block (RB) bundling” (i.e., bundling contiguous RBs together to perform channel estimation and demodulation) will help higher rank (i.e., rank 5 to 8) transmissions achieve adequate channel estimation accuracy along with low overhead. It is also noted that RB bundling could be used to balance the transmission power imbalance across OFDM symbols for some high rank DM-RS patterns, as disclosed in 3GPP Document No. R1-094575, “Discussion On DM-RS For LTE-Advanced”, Samsung (November 2009); 3GPP Document No. R1-094438, “On Rel-10 DM RS Design For Rank 5-8”, Ericsson, ST-Ericsson (November 2009), and 3GPP Document No. R1-094548, “Further Investigation On DMRS Design For LTE-A”, CATT (November 2009), which are hereby incorporated by reference into the present disclosure as if fully set forth herein.
FIGS. 3A-3C illustrate dedicated reference signal (DRS) patterns that support two and four layer transmissions according to an embodiment of this disclosure. Dedicated reference signal (DRS) patterns 301 and 303 illustrate pilot patterns that can support up to two (2) layer transmissions. DRS resource elements labeled with (0,1) in DRS pattern 301 carry dedicated reference signals for layer 0 and 1 with the reference signals of the two layers code-division multiplexed (CDMed). Similarly, for DRS resource elements labeled with (2,3) in DRS pattern 303 carry dedicated reference signals for layer 2 and 3 with the reference signals of the two layers code-division multiplexed (CDMed).
In the two adjacent DRS resource elements labeled with (0,1), DRS symbols [r0 r1] for layer 0 are mapped to the two resource elements spread by a Walsh code [1 1], which results in [r0 r1], while DRS symbols r2 and r3 for layer 1 are mapped to the two resource elements spread by a Walsh code [1 −1], which results in [r2 −r3].
DRS pattern 305 illustrates a pilot pattern that can support up to four layer transmissions, where the DRS resource elements are again partitioned into two groups, those labeled with (0,1) and those with (2,3). In this pattern, the DRS resource elements labeled with (0,1) carry dedicated reference signals for layer 0 and 1 with the reference signals of the two layers code-division multiplexed (CDMed). The DRS resource elements labeled with (2,3) carry dedicated reference signals for layer 2 and 3 with the reference signals of the two layers code-division multiplexed (CDMed).
FIG. 4 illustrates DRS patterns 401 and 403, which support eight layer transmissions according to an embodiment of the disclosure. In FIG. 4, resource elements labeled with alphabet character X, where X is one of G, H, I, J, L, K, are used for carrying a number of dedicated reference signals among the 8 dedicated reference signals, where the number of dedicated reference signals are CDMed. DRS pattern 401 is based on spreading factor 2 CDM across two time-adjacent resource elements with the same alphabet character label. DRS pattern 403 is based on spreading factor 4 CDM across two groups of two time-adjacent resource elements with the same alphabet character label. In this embodiment, the 8 antenna ports in a Rank-8 pattern are referred to as antenna ports 4, 5, 6, 7, 8, 9, 10 and 11 in the sequel to distinguish them from the antenna ports in Rank-2 and Rank-4 patterns.
It is noted that in Rel-8 LTE, antenna ports 0, 1, 2, 3, 4 and 5 are used for CRS, MBSFN RS and Rel-8 DRS. Hence, if the numbering convention extending Rel-8 LTE is followed, the new antenna port numbers will start from 6. Rank-2 pattern will have antenna ports (6, 7). Rank-4 pattern will have antenna ports (7, 8, 9, 10). Rank-8 pattern will have antenna ports (11, 12, 13, 14, 15, 16, 17, 18).
In one embodiment of DRS pattern 401, G carries DRS (4, 5), H carries DRS (6,7), I carries DRS (8,9) and J carries DRS (10,11). In one embodiment of DRS pattern 403, K carries DRS (4, 5, 6, 7) and L carries DRS (8, 9, 10, 11).
Each of the demodulation reference signal (DM-RS) patterns in FIGS. 3A-3C and FIG. 4 is resource block (RB) based. Accordingly, a UE (or MS) may perform channel estimation and demodulation per resource block. Alternatively, if resource block bundling is supported, the UE (or MS) may perform channel estimation and demodulation jointly across bundled resource blocks. In this way, the performance of channel estimation and demodulation can be improved.
Resource block-bundling gain is achieved only when a base station (BS or eNodeB) performs the same downlink precoding vectors across the bundled resource blocks. Accordingly, a UE or MS may perform channel estimation and demodulation over the bundled resource blocks jointly.
In other words, resource block bundling reduces the precoding flexibility, since the precoding vectors within the bundled resource blocks have to be the same. This results in a trade-off between gains from increasing channel interpolation span in frequency versus losses from increasing frequency selective precoding granularity.
Therefore, there is a need for improved techniques for bundling resource blocks in a wireless communication system.